Preserved thinking: changing how the Messages API handles thinking blocks to protect against distillation | Claude Help Center
AI-generated summary The support notice explains that the Messages API handles thinking blocks differently to protect against distillation. The change affects models including Claude Fable 5.1, Claude Opus 5.5, Claude Sonnet 5.5, Claude Haiku 5.5, Claude Fable 5.2, and Claude Mythos 5.2.